This invention relates to a driving circuit for an electro-absorption optical modulator. More particularly, the invention relates to a driving circuit for an electro-absorption optical modulator for outputting intensity-modulated signal light by receiving carrier light from a light source and absorbing the carrier light in dependence upon driving voltage.
A light transmitter intensity-modulates carrier light, which is output by a laser diode serving as a light source, in dependence upon the "1", "0" logic of a data signal and sends the intensity-modulated signal light to an optical transmission line. An electro-absorption optical modulator (referred to as an "EA modulator) is available as an external modulator for performing the intensity modulation. The EA modulator generates intensity-modulated signal light by absorbing carrier light in dependence upon applied voltage (i.e., driving voltage).
FIG. 4 is a block diagram showing the general construction of a light transmitter using an EA modulator as the optical modulator. The transmitter includes a laser diode (LD) 1 as the light source, a laser diode drive 2 for causing the laser diode to emit light at a constant intensity and including an ACC (Automatic Current Control) circuit, which regulates laser diode driving current to a constant current value, and an ATC (Automatic Temperature Control) circuit for regulating laser diode chip temperature to a constant temperature, an EA modulator 3 for intensity-modulating carrier light from the laser diode by the applied voltage (driving voltage) to effect a conversion to signal light, a modulating signal generating circuit 4 for outputting a pulsed modulating signal based upon "1"s and "0"s of the data signal (input signal IN) and applying the pulsed driving voltage to the EA modulator, an isolator 5 for sending the signal light, which is output by the EA modulator, to an optical fiber 6, and a terminating resistor 7 for applying the pulsed driving voltage to the EA modulator 3 on the basis of the pulsed modulating signal. The resistor 7 is for impedance matching with the modulating signal generating circuit 4 and is assumed to have a resistance of, e.g., 50 ohms.
The modulating signal generating circuit 4 is constituted by a differential-type switch and a constant-current source. More specifically, as shown in FIG. 5, the modulating signal generating circuit 4 includes FETs 4a, 4b in a differential pair whose source terminals are tied together and connected to a constant-current source 4c. The drain terminal of the FET 4a is connected to ground via a resistor R, the drain terminal of the FET 4b is connected to the EA modulator 3, an input signal IN is applied to the gate terminal of the FET 4a, and a signal *IN, which is the inverse of the input signal IN, is applied to the gate terminal of the FET 4b. If the input signal IN is logical "1", the FET 4a turns on, the FET 4b turns off at the same time and a constant current flows into the FET 4a via the resistor R. If the input signal IN is logical "0", the FET 4a turns off, the FET 4b turns on at the same time and a constant current flows into the FET 4b. Thus, a pulsed modulating signal is generated based upon the "1", "0" logic levels of the input signal. If FIGS. 4 and 5 are expressed in simplified form, the result is as shown in FIG. 6 (in which the isolator and optical fiber have been deleted).
FIG. 7 is a diagram useful in describing the operation of the circuit. A characteristic curve 11 represents the relationship between output power P.sub.0 of the EA modulator 3 and voltage V.sub.EA applied to the EA modulator 3. It will be appreciated that the output power P.sub.0 is approximately inversely proportional to the square of the applied voltage V.sub.EA. If the applied voltage V.sub.EA is pulse-modulated at values at which the output power P.sub.0 is maximized and at values at which the output power P.sub.0 is approximately minimized, as indicated by the solid line 12 in FIG. 7, then a signal light intensity modulated as indicated at 13 is output by the EA modulator 3.
FIG. 8 shows an electro-absorption static characteristic illustrating the relationship between a photocurrent I.sub.PH, which is produced by the EA modulator 3 owing to absorption of carrier light, and the applied voltage V.sub.EA, as well as the relationship between the output power P.sub.0 and the applied voltage V.sub.EA. Here the applied voltage V.sub.EA is plotted along the horizontal axis while the signal light output P.sub.0 and photocurrent I.sub.PH are plotted along the vertical axis. As described above in connection with FIG. 7, the output power P.sub.0 of the signal light varies as indicated by a curve 11 as the applied voltage V.sub.EA varies. Further, the photocurrent I.sub.PH developed in the EA modulator 3 increases as indicated by curve 21 as the output power P.sub.0 of the signal line decreases (i.e., as the amount of absorption of the carrier light increases). That is, if the applied voltage V.sub.EA is increased, light is absorbed, the output power P.sub.0 decreases and the absorbed light appears as the photocurrent I.sub.PH.
It should be evident from the electro-absorption static characteristic of FIG. 8 that photocurrent I.sub.PH is not linear in the relationship with the applied voltage V.sub.EA. The photocurrent I.sub.PH presents a linear characteristic in the region in which the EA voltage V.sub.EA is less than V.sub.EATH (i.e., in the low-voltage region) and a saturated characteristic in the region in which the EA voltage V.sub.EA is greater than V.sub.EATH (i.e., in the high-voltage region). If this is considered in terms of the impedance of the EA modulator, the impedance is low in the low-voltage region where the EA voltage V.sub.EA is less than V.sub.EATH and high in the high-voltage region where the EA voltage V.sub.EA is greater than V.sub.EATH.
The 50-ohm terminating resistor is connected in parallel with the EA modulator 3, as shown in FIGS. 4 through 6, and the EA modulator 3 performs pulse modulation while impedance is matched with that of the modulating signal generating circuit 4 by this resistor. If the EA modulator 3 is operating at high impedance, then impedance matching is achieved correctly. If the EA modulator 3 operates at low impedance, however, the impedance as seen from the modulating signal generating circuit 4 declines, impedance mismatching occurs and the waveform deteriorates.
FIGS. 9A and 9B are diagrams useful in describing waveform deterioration in a case where the aforesaid impedance mismatch has occurred. Here a pulsed current source Ip is terminated at an impedance Z.sub.L through a transmission line having an impedance Z.sub.0 and exhibiting a propagation delay time of .tau..sub.pd. The conditions which prevail when Z.sub.L &lt;Z.sub.0 holds will be considered. The Z.sub.L &lt;Z.sub.0 state represents the state of mismatched impedance. As a consequence, reflection is produced by the impedance Z.sub.L and by the pulsed current source Ip, as indicated by the waveforms (2), (3) in FIG. 9B, and the waveform of the voltage across the impedance Z.sub.L is as indicated by the waveform (4). Specifically, if impedance mismatch occurs, the waveform of the voltage cross the impedance Z.sub.L has steps on both sides owing to reflection.
The EA modulator 3 produces impedance mismatch if it operates at low impedance, as mentioned above. Consequently, a problem which arises is that an EA voltage waveform 12 whose waveform has deteriorated in the manner shown in FIG. 10 is applied to the EA modulator 3 so that the EA modulator 3 outputs signal light 13 the positive-going transition of which has a deteriorated waveform.